1. Field of the Invention
The present invention relates to a semiconductor light-emitting device using a nitride-based III-V group compound semiconductor.
2. Description of the Background Art
Recently, as a semiconductor laser which enables light emission in a range from a blue region to an ultraviolet region, which is required for making the density of an optical disc high, a nitride-based semiconductor laser using a nitride-based III-V group compound semiconductor has been intensively researched and developed, and has been already put in use.
In the nitride-based semiconductor lasers reported so far, as an active layer structure, there has been frequently adopted a multiple quantum well structure with two or more well layers made of InGaN and three or more barrier layers made of InGaN having a smaller In composition ratio than that of the InGaN of the well layers (usually about 0.02) are laminated alternately.
In the nitride-based III-V group compound semiconductor, the ratio of the band discontinuity of the conduction band and the band discontinuity of the valence band when a hetero junction is formed is about 2.5:7.5, and it is known that the band discontinuity of the conduction band is very small (see Applied Physics Letter vol. 70 (1997), p. 2577). Accordingly, electrons are likely to overflow from the well layer beyond the barrier layer, which often leads to problems such as increase in threshold current, and deterioration of differential efficiency and temperature characteristic.
In order to solve these problems, it may be considered to increase the band discontinuity of the conduction band by widening the band gap of the barrier layer as much as possible. For this purpose, InGaN having smaller In composition ratio may be used as the barrier layer, or a material having larger band gap than that of InGaN may be used, such as GaN, AlGaN or InAlGaN.
When the band gap of the barrier layer is widened, the band discontinuity of the valence band is increased. However, as a barrier layer, when InGaN having the smallest In composition ratio is used or when GaN or AlGaN is used, these materials become smaller in lattice constant as they become wider in band gap, and are exposed to greater tensile distortion. According to the band properties of semiconductors receiving this distortion, the band discontinuity of the valence band is not increased so much as compared with the increase of the band discontinuity of the conduction band.
As a result, it is considered that the problem of overflow of electrons can be solved without causing a problem in uniform injection of holes in the wells of two layers or more.
The conventional art related to the present invention is disclosed in Japanese Patent Application Laid-Open No. 07-170022 (1995).
However, the present inventor has confirmed by simulation that a new problem arises when a barrier layer is made of a material having larger band gap, electrons are likely to overflow beyond the electron blocking layer from a final barrier layer which is closest to the p-side. It is considered that this problem is caused due to the following reason: the barrier layer is made of a material having a larger band gap, so that a band discontinuity (electron barrier) between the final barrier layer and the electron blocking layer becomes small.
Further, in the case of barrier layers of InGaN of smaller band gap than GaN or AlGaN, the rate of electrons overflowing the electron barrier from the final barrier layer is not zero, which may lead to deterioration of threshold current of a semiconductor light-emitting device, differential efficiency, and temperature characteristics.
Accordingly, omitting the final barrier layer, by bonding the well layer and electron blocking layer directly, may be considered to reduce the overflowing electrons by increasing the height of the electron barrier, instead of the presence of a final barrier layer.
However, the film forming temperature of an electron blocking layer is higher than that of a well layer by about 200° C. Therefore, if it is attempted to form an electron blocking layer directly on the well layer, after forming the well layer, the film forming temperature must be raised without protecting the surface of the well layer. In this process, crystallinity of the well layer surface deteriorates.
When a semiconductor light-emitting device is manufactured by using such a well layer deteriorated in crystallinity, electrons trapped in the well layer do not contribute to laser oscillation, and threshold current or differential efficiency may be worsened.